1. Field of Invention
The present invention relates to a switching regulator and control method thereof, and a method for determining On-time in switching regulator; particularly, it relates to a method for determining On-time in a pulse skipping mode (PSM) without an input or output voltage signal.
2. Description of Related Art
There are many types of switching regulators, such as buck converter, boost converter, inverter converter, and buck-boost converter, etc. By way of example, FIG. 1 shows a conventional buck switching regulator 1 comprising two transistors Q1 and Q2, which are controlled by a control circuit 10. The control circuit 10 generates switching control signals DRV1 and DRV2 according to a feedback signal FB obtained from the output Vout, and the transistor switches Q1 and Q2 are controlled by the control signals to convert electrical power from an input Vin to an output Vout. FIGS. 2-4 show a boost converter 2, an inverter converter 3, and a buck-boost converter 4, respectively. Though the circuit architecture and the number of transistors are different, these circuits operate under the same mechanism: The control circuit 10 generates control signals according to the feedback signal FB, and the power transistor switches operate to convert the electrical power from the input Vin to the output Vout according to the control signals. In these circuits, one of the power transistor switches can be replaced by a diode to constitute an asynchronous switching regulator.
In the aforementioned switching regulators, there is a light load efficiency issue if the power transistor switch still operates in the pulse width modulation (PWM) mode in light load condition (“light load” means that the load current, i.e., current supplied to a load circuit, is relatively low). Taking a synchronous buck converter as an example, when the load current is declining, and the operation mode is still the continuous conduction mode (CCM), the electrical power is back to the input from the output once the lower bridge current is less than zero. A power conversion inefficient issue occurs in consequence. Thus, as the inefficient issue occurs, it is indispensable to operate the lower bridge circuit in a mode like a non-synchronous diode mode, a so-called diode emulated mode, such that the regulator can enter a discontinuous conduction mode (DCM). When the regulator enters DCM from CCM, the COMP voltage decreases by following the decline of the load current, and then a duty signal pulse width decrease as well. Therefore, when the load current is extremely low, due to the pulse width too narrow, the electrical power converted from the input to the output will be extremely limited. Meanwhile, the power switching loss of operating the upper and lower bridge transistors each time remains nearly unchanged. And then, when the load current is extremely low, it is appropriate to maintain a minimum On-time of the power transistor switch but reducing the operation frequency, i.e., a pulse skipping mode. It is very important how to determine On-time in the pulse skipping mode. But On-time relates to the application environment (like input voltage and output voltage) of the regulator. It is appropriate to set On-time according to the environment of the regulator. If On-time is too long, the ripple of the voltage will be too large; if too short, the efficiency improvement will be limited.
In view of foregoing, U.S. Pat. No. 7,456,624 discloses a method and circuit for entering the pulse skipping mode as shown in FIG. 5. This prior art does not disclose the circuit details. According to the specification and figures, this prior art needs the input voltage Vin and the output voltage Vout to determine whether entering the pulse skipping or not mode and On-time in the pulse skipping mode. The drawback of this prior art is that, there is an essential requirement of two extra pins of the integrated circuit (IC, in general, the control circuit 10 is manufactured to an IC) to obtain the Vin and Vout signals. Not only extra two pins are essentially required in the IC, but it also can cause an inaccurate issue due to an error of handling the two signals. This prior art have obstacles in the circuit design and embodiment.
In view of the above drawbacks, it is necessary to introduce a method, and a relating switching regulator and a control method therefor that can determine On-time in the pulse skipping mode without the input voltage and the output voltage.